1. Field of the Invention
The present invention relates to compositions and methods for drilling boreholes in subterranean formations, particularly hydrocarbon bearing formations, and to drilling fluids for use in such drilling operations. More particularly, the present invention relates to oil and synthetic fluid based drilling fluids comprising invert emulsions, and drilling fluid additives that enhance suspension characteristics of such drilling fluids.
2. Description of Relevant Art
A drilling fluid or mud is a specially designed fluid that is circulated through a wellbore as the wellbore is being drilled to facilitate the drilling operation. The various functions of a drilling fluid include removing drill cuttings from the wellbore, cooling and lubricating the drill bit, aiding in support of the drill pipe and drill bit, and providing a hydrostatic head to maintain the integrity of the wellbore walls and prevent well blowouts. Specific drilling fluid systems are selected to optimize a drilling operation in accordance with the characteristics of a particular geological formation. As used herein, the term “drilling fluid” shall be understood to include fluids used in drilling, and/or cementing, casing and/or other completion operations downhole.
Oil or synthetic fluid-based muds are normally used to drill swelling or sloughing shales, salt, gypsum, anhydrite or other evaporate formations, hydrogen sulfide-containing formations, and hot (greater than about 300 degrees Fahrenheit (“° F.”) holes, but may be used in other holes penetrating a subterranean formation as well. Unless indicated otherwise, the terms “oil mud” or “oil-based mud or drilling fluid” shall be understood to include synthetic oils or other synthetic fluids as well as natural or traditional oils, and such fluids shall be understood to comprise invert emulsions. As used herein, the term “synthetic fluid” has the meaning generally known and used by the oil and gas drilling industry. Generally, the term “synthetic fluid” means a material produced by the chemical reaction of specific purified chemical feedstock as opposed to traditional base fluids such as diesel and mineral oil which are derived from crude oil solely through physical separation processes, e.g., fractionation, distillation, and minor processes such as cracking and hydroprocessing.
Oil-based muds and synthetic-fluid based muds used in drilling typically comprise: a base oil or synthetic fluid comprising the external phase of an invert emulsion; a saline, aqueous solution (typically a solution comprising about 30% calcium chloride) comprising the internal phase of the invert emulsion; emulsifiers at the interface of the internal and external phases; and other agents or additives for suspension, weight or density, oil-wetting, fluid loss or filtration control, and rheology control. Such additives commonly include organophilic clays and organophilic lignites. See H. C. H. Darley and George R. Gray, Composition and Properties of Drilling and Completion Fluids 66-67, 561-562 (5th ed. 1988). An invert emulsion-based drilling fluid may commonly comprise between about 50:50 to about 95:5 by volume oil phase to water phase, or synthetic fluid phase to water phase. An all oil mud simply comprises 100% liquid phase oil by volume; that is, there is no aqueous internal phase.
Invert emulsion-based muds or drilling fluids (also called invert drilling muds or invert muds or fluids) comprise a key segment of the drilling fluids industry. However, increasingly invert emulsion-based drilling fluids have been subjected to greater environmental restrictions and performance and cost demands. With space at some well sites limited, such as on offshore platforms, and with increasing costs of transport of materials to a wellsite, there is industry wide interest in, and on-going need for, more efficient drilling fluid additives and for drilling fluids which can be formulated and maintained with minimal or fewer additives than common with prior art drilling fluids, while still affording environmental and economical acceptance.